1. Field of the Invention
This invention relates to new and improved catalysts for the production of vinyl acetate by reaction of ethylene, oxygen and acetic acid.
2. Background Information Including Description of Related Art
It is known to produce vinyl acetate by reaction of ethylene, oxygen and acetic acid using a catalyst consisting of metallic palladium and gold supported on a carrier. While the process utilizing such a catalyst is capable of producing vinyl acetate at fair levels of productivity, such productivity levels are limited by the production of undesirable by-products, particularly carbon dioxide. Thus, any expedient capable of achieving reduced production of by-products such as carbon dioxide, expressed as lower percent CO2 selectivity, is very desirable.
The foregoing catalysts comprising metallic palladium and gold are conventionally prepared by a process including the steps of impregnating a porous support with a single aqueous solution or separate solutions of water-soluble salts of palladium and gold, reacting the impregnated water-soluble salts with an appropriate alkaline compound e.g., sodium hydroxide, to xe2x80x9cfixxe2x80x9d the palladium and gold as water-insoluble compounds, e.g. the hydroxides, and reducing the water insoluble compounds, e.g., with ethylene or hydrazine, to convert the palladium and gold to free metallic form. This type of process has the disadvantage of requiring several steps, sometimes including at least two xe2x80x9cfixingxe2x80x9d steps.
The following references may be considered material to the invention claimed herein.
U.S. Pat. No. 5,332,710, issued Jul. 26, 1994, to Nicolau et al., discloses a method of preparing a catalyst useful for the production of vinyl acetate by reaction of ethylene, oxygen and acetic acid, comprising impregnating a porous support with water soluble salts of palladium and gold, fixing the palladium and gold as insoluble compounds on the support by immersing and tumbling the impregnated support in a reactive solution to precipitate such compounds, and subsequently reducing the compounds to free metallic form.
British Patent No. 1,188,777, published Apr. 22, 1970 discloses a process for the simultaneous production of an unsaturated carboxylic acid ester, e.g. vinyl acetate, from an olefin, carboxylic acid, and oxygen, and the corresponding carboxylic acid, e.g., acetic acid, from its aldehyde, using a single supported catalyst containing a palladium compound, e.g. an oxide or salt, with one or more compounds of any of various metals, e.g. metallic gold or a gold compound such as potassium aurate.
U.S. Pat. No. 5,700,753 discloses vinyl acetate (VA) catalyst prepared by adding organometallic gold complexes to prereduced palladium catalyst prepared from Na2PdCl4. The organometallic gold compound does not require a fixing procedure.
In accordance with this invention, a catalyst is provided useful for the production of vinyl acetate (VA) by reaction of ethylene, oxygen and acetic acid with low carbon dioxide selectivity, said catalyst being prepared by steps comprising impregnating a porous support, the porous surfaces of which contain a catalytically effective amount of prereduced metallic palladium, with a solution of potassium aurate, KAuO2, and reducing the potassium aurate to a catalytically effective amount of metallic gold. The use of such catalyst often results in lower carbon dioxide and heavy ends selectivities, which are usually accompanied by a higher vinyl acetate productivity, than when various conventional catalysts comprising metallic palladium and gold are employed.
Disclosed herein is a process to prepare catalyst useful in the production of VA. Prereduced Pd catalyst were prepared by the impregnation of a support with aqueous solution of Na2PdCl4 followed by fixing with NaOH and reduction of the Pd. A thin shell Pd catalyst was obtained, to which was then contacted with a solution of aqueous KAuO2 to form a second shell of Au on the support. Ultimately, a shell catalyst of Pd and Au was formed wherein a fixing step for the Au was not necessary. The Pd and Au were distributed as a thin metal shell on the support structure.
As an alternative embodiment, the catalyst may be prepared by first contacting the support with KAuO2, followed by contact with Na2PdCl4. The Pd compound may then be fixed with a precipitating solution such as NaOH, and the Au and Pd reduced with a reducing agent. Alternatively, the Au may be reduced before the addition of the Pd solution.
A still further embodiment of the inventive procedure involves use of sodium-free reagents; such as described in U.S. Pat. No. 5,693,586.
The support material in the catalyst of this invention is composed of particles having any of various regular or irregular shapes, such as spheres, tablets, cylinders, rings, stars, or other shapes, and may have dimensions such as diameter, length or width of about 1 to about 10 mm., preferably about 3 to 9 mm. Spheres having a diameter of about 4 to about 8 mm. are preferred. The support material may be composed of any suitable porous substance, e.g., silica, alumina, silica-alumina, titania, zirconia, silicates, aluminosilicates, titanates, spinel, silicon carbide, carbon, and the like.
The support material may have a surface area within the range, for example, of about 10 to about 350, preferably about 100 to about 200 m2/g, an average pore size in the range, for example, of about 50 to about 2000 angstroms, and a pore volume in the range, for example, of about 0.1 to about 2, preferably about 0.4 to about 1.2 ml/g.
In the preparation of the catalyst used in the process of this invention, the support material is first treated to deposit a catalytic amount of palladium on the porous surfaces of the support particles. Any of various methods for accomplishing this purpose may be used, all of which involve the impregnation of the support with an aqueous solution of a water-soluble compound of palladium. Palladium(II) chloride, sodium palladium(II) chloride (i.e., sodium tetrachloropalladium(II), Na2PdCl4), potassium palladium(II) chloride, palladium(II) nitrate or palladium(II) sulfate are examples of suitable water-soluble palladium compounds. Sodium tetrachloropalladium(II) is the preferred salt for impregnation because of its good water solubility. The impregnation can be accomplished by the xe2x80x9cincipient wetnessxe2x80x9d method wherein an amount of water-soluble metal compound solution used for the impregnation is from about 95 to about 100 percent of the absorptive capacity of the support material. The concentration of the solution is such that the amount of elemental palladium in the solution absorbed on the support is equal to a desired predetermined amount. The impregnation is such as to provide, for example, about 1 to about 10 grams of elemental palladium per liter of finished catalyst.
After the impregnation of the support with an aqueous solution of water-soluble salt of palladium, the palladium is xe2x80x9cfixedxe2x80x9d, i.e., precipitated, as a water-insoluble compound such as the hydroxide, by reaction with an appropriate alkaline compound, e.g., an alkali metal hydroxide, silicate, borate, carbonate or bicarbonate, in aqueous solution. Sodium and potassium hydroxides are preferred alkaline fixing compounds. The alkali metal in the alkaline compound should be in an amount of, for example, about 1 to about 2, preferably about 1.1 to about 1.8 times the amount necessary to react with the catalytically active cations present in the water-soluble salt. The fixing of the palladium may be done by the incipient wetness method wherein the impregnated support is dried, e.g., at a temperature of 150xc2x0 C. for one hour, contacted with an amount of solution of the alkaline material equal to about 95-100% of the pore volume of the support, and allowed to stand for a period of about xc2xd to about 16 hours; or the roto-immersion method wherein the impregnated support without drying is immersed in a solution of the alkaline material and is rotated and/or tumbled during at least the initial period of precipitation such that a thin band of the precipitated water-soluble compound is formed at or near the surface of the support particles. The rotation and tumbling may be carried out, for example, at about 1 to about 10 rpm for a period, e.g., of at least about 0.5 hour, preferably about 0.5 to about 4 hours. The contemplated roto-immersion method is disclosed in U.S. Pat. No. 5,332,710, the entire disclosure of which is incorporated herein by reference.
The fixed, i.e., precipitated palladium compound may then be reduced, for example, in the vapor phase with ethylene, e.g., 5% in nitrogen at 150xc2x0 C. for 5 hours, after first washing the catalyst containing the fixed palladium compounds until it is free of anions such as halide, and drying, e.g., at 150xc2x0 C. overnight under constant N2 purge, or such reduction may be accomplished in the liquid phase at room temperature with an aqueous solution of hydrazine hydrate wherein the excess of hydrazine over that required to reduce all the metal compounds present on the support is in the range, for example, of about 8:1 to about 15:1, followed by washing and drying. Other reducing agents and means for reducing the fixed palladium compound present on the support may be employed as conventional in the art. The reduction of the fixed palladium compound mainly results in the formation of the free metal, although a minor amount of metal oxide may also be present.
After the catalyst containing palladium in free metallic form deposited on a support material is prepared by any of the foregoing methods, it is impregnated with an aqueous solution of potassium aurate, preferably by incipient wetness. The catalyst is then dried such that the catalyst contains potassium aurate in an amount sufficient to provide, for example, about 0.5 to about 10 grams of elemental gold per liter of finished catalyst, with the amount of gold being from about 10 to about 125 wt. % based on the weight of palladium present. The potassium aurate is then reduced to metallic gold using any of the techniques described previously for the reduction of palladium from the xe2x80x9cfixedxe2x80x9d, i.e., water insoluble, palladium compound on the surface of the support. Such reduction of potassium aurate is carried out without any necessity for the intermediate steps of fixing the gold on the support as a water-insoluble compound and washing such compound until chlorine-free, as described previously for palladium and as ordinarily required for gold in the preparation of vinyl acetate catalysts comprising palladium and gold. The elimination of such fixing and washing steps in connection with gold is an important advantage in the preparation of the catalyst of this invention. A high gold metal retention catalyst was obtained by this method. The catalyst also contains Pd and Au distributed in a thin shell at or near the surface of the catalyst support.
One of the problems in producing VA catalysts has been low noble metal retention on the catalyst support. The use of KAuO2 precursors offer a method to produce salt-free, highly dispersed metallic particle catalysts, with no fixing step involved for the Au complexes. An advantage of no fixing step for the Au complexes is the increased gold retention since Au is partially washed out of the catalyst during the fixing/washing step under prior art techniques, and increased Au/Pd ratio on the catalyst.
Although the catalysts of this invention have been described primarily in connection with those containing only palladium and gold as catalytically active metals, the catalyst may also contain one or more additional catalytically active metallic elements in the form of the free metal, oxide, or mixture of free metal and oxide. Such metallic elements may be, for example, copper, magnesium, calcium, barium, zirconium and/or cerium. When a metal in addition to palladium and gold is desired in the catalyst, the support may usually be impregnated with a water soluble salt of such metal dissolved in the same impregnating solution as that containing the water-soluble palladium salt. The support may thus be simultaneously impregnated with water-soluble salts of palladium and the additional metal which are then simultaneously fixed and reduced in the same manner as described previously for palladium alone. The catalyst containing the palladium as the free metal and an additional metal as the oxide and/or free metal is then impregnated with potassium aurate which is then reduced to gold as free metal without an intermediate fixing step as described previously in connection with palladium as the only other metal in addition to gold.
Advantageously, the catalyst containing palladium and gold in free metallic form may optionally be impregnated with a solution of an alkali metal acetate, preferably potassium or sodium acetate, and most preferably potassium acetate (KOAc). After drying, the finished catalyst may contain, for example, about 10 to about 70, preferably about 20 to about 60 grams of alkali metal acetate per liter of finished catalyst. Optimally, KAuO2 may be added together with KOAc in one step to the prereduced Pd catalyst.
When vinyl acetate is prepared using the catalyst of the present invention, a stream of gas, which contains ethylene, oxygen or air, acetic acid, and desirably an alkali metal acetate, is passed over the catalyst. The composition of the gas stream can be varied within wide limits, taking in account explosive limits. For example, the molar ratio of ethylene to oxygen can be about 80:20 to about 98:2, the molar ratio of acetic acid to ethylene can be about 2:1 to about 1:10, preferably about 1:2 to 1:5, and the content of gaseous alkali metal acetate can be about 1 to about 100 ppm based on the weight of acetic acid employed. The gas stream also can contain other inert gases, such as nitrogen, carbon dioxide and/or saturated hydrocarbons. Reaction temperatures which can be used are elevated temperatures, preferably those in the range of about 150-220xc2x0 C. The pressure employed can be a somewhat reduced pressure, normal pressure or elevated pressure, preferably a pressure of up to about 20 atmospheres gauge.